In line with recent developments of information communication equipment such as space communication and automobile telephones, there is a strong demand for dielectric resonators for use in generating microwaves. In addition, there is a demand for compact, high efficient and inexpensive resonators.
FIGS. 14(A) and (B) show a typical example of a known dielectric resonator, the former showing a perspective view and the latter showing a cross-sectional view. The configuration common to the known resonators is cylindrical as shown in FIGS. 14(A), (B), but the configuration can vary such as rectangular and polygonal. The cylindrical configuration is particularly advantageous in that it ensures an excellent spuriaus effect. In FIGS. 14(A), (B) the resonator is composed of a body 1 of dielectric ceramics having a bore 2 and an electrode 3.
The resonator is produced first by molding a raw dielectric ceramic into the body 1 of a desired shape and sintering it at an elevated temperature. The body 1 including the inside surface of the bore 2 is wholly or partly coated with a conductive pasty mixture of silver powder and glass frit, and then is sintered at a high temperature between 600.degree. to 800.degree. C., thereby producing the electrode 3 in a film having a thickness of 10 to 20.mu.. Recently, in order to reduce the production cost and speed up production, an electroless plating method is directly applied to the body 1, thereby producing the electrode 3.
However, the former resonator is likely to be expensive because of the silver component. In addition, the Q value drops owing to the interposition of the glass between the silver and the body 1. What is more, it is difficult to evenly coat the inside surface of the bore 2, thereby preventing mass production.
The electroless plating process is disclosed in Japanese Laid-Open Patent Publication No. 54-108544.
The disadvantage of the electroless plating is that the body 1 is subjected to a lot of bulges because of weak bond between the body 1 and the copper film.
In order to solve the problem of bulging, one proposal is that the copper film is heat treated in an inert gas such as Ni or Ar (Japanese Laid-Open Patent Publication No. 58-166806). Another proposal is that prior to applying the electroless plating, the surface of the body 1 is roughened with an acid mixture containing a degreasing agent and hydrofluoric acid, and then the copper film formed thereon by the electroless plating is heat treated at a reducing atmosphere or at a weak acid atmosphere (Japanese Laid-Open Patent Publication No. 61-121501).
The heat treatment at an inert gas atmosphere, a reducing atmosphere and a weak acid atmosphere may improve the strength of bond but thermal-shock tests have uncovered that the resulting resonators are liable to bulging on the plated film, wherein the thermal-shock tests were conducted about 100 cycles under hard conditions (-60.degree. to +115.degree. C., each temperature being maintained for 30 minutes), and that the Q value of the resonator decreases. Presumably the detrimental bulging and reduction in the Q value results from the fact that the heat-impact test weakens the bond between the plated film and the body 1.